A typical use for an encoder is to determine the rotational velocity and/or position of a rotating object, typically a shaft. In certain applications, the need arises to ensure that the encoder has not failed during operation and so error detection is needed. Otherwise, the functional failure of a shaft encoder can cause abnormal behaviors of the machine in which the shaft operates such as shaft speed vibration, wrong rotating direction, undesirable acceleration, and other concerns. The abnormalities can then result in processed parts out of tolerance, damaged parts, as well as wrong direction rotation for moving parts on machines.
One solution is disclosed in U.S. Pat. No. 4,597,081 wherein an optical encoder interface performs error checking on each full revolution of the encoder. The pulses per revolution are counted and compared to a reference number that should occur in one revolution with a properly operating system. This error detection operates on an encoder with a three channel signal where an index revolution bit (a Z-bit) is employed to determine when a complete revolution has occurred. The error detection capability is implemented in the hardware of the electronic assembly itself. However, this solution has limitations that are not always desirable. Such as, the error detection is built into the hardware itself, thus limiting its adaptability and use in retrofitting existing assemblies. Also, this error detection assembly requires a three channel signal, and thus cannot be employed with two channel encoder applications. This adds to the cost and complexity if one wishes to operate an encoder with this error detection. And further, the error detection determination can only be made once every revolution of the shaft, thus limiting how quickly the error will be detected.
Consequently, it is desired to have a system with an encoder that accomplishes error detection but does not have the above noted drawbacks.